1. Field of the Invention
The present invention generally relates to an optical disk apparatus using a flexible sheet-like optical disk as an optic information recording medium and having a stabilizing unit therein which stabilizes surface vibration in a rotary axial direction of the optical disk by using pressure difference of air flow at a portion of the optical disk at which writing/reading is performed, and a method of controlling the optical disk apparatus, and particularly relates to a technique for adjusting a positional relation between the optical disk and the stabilizing member.
2. Description of the Related Art
Optical disks are demanded to store mass digitized data in response to, for example, the recent digitization of television broadcasting. A basic method of improving recording density on the optical disks is to reduce the diameter of a beam spot used for recording/reproducing information onto/from the optical disk.
For this reason, it is effective to shorten the wavelength of light used for recording/reproduction, and also, it is effective to enlarge the numerical aperture NA of an objective lens. As for the wavelength of light, the wavelength of approximately 650 nm of red light is used on a DVD (digital versatile disk) while 780 nm of near infrared light is used on a CD (compact disk). Recently, a semiconductor laser of blue-violet light has been developed and it is expected that a laser light of approximately 400 nm will be used.
The objective lens for a CD has less than 0.5 NA while the objective lens for a DVD has approximately 0.6 NA. The numerical aperture (NA) is demanded to be enlarged further to 0.7 or more in the future. However, enlarging NA of the objective lens and shortening the wavelength of light may result to an increase in influence of aberration in a case where applied light is weakened. Therefore, tilt margin of the optical disk may decrease. Moreover, since the depth of focus becomes smaller by enlarging NA, it will be necessary to increase focus servo accuracy of the optical disk apparatus.
Furthermore, since the distance between the objective lens and recording surface of an optical disk becomes shorter by using the objective lens of high NA, the objective lens and optical disk may collide before the focus servo becomes fully effective during the start-up period, unless surface vibration or axial runout on the disk is sufficiently controlled.
“O PLUS E” (vol. 20, No. 2, page 183) proposes an example of a large-capacity optical disk drive system having a short wavelength and high NA, in which a recording film is formed on a substrate having an equal thickness and an equal rigidity as a CD, and light for recording/reproducing is allowed to pass not through the substrate but through a thin cover layer for recording/reproducing on/from the recording film.
Japanese laid-open patent publication No. 7-105657 and Japanese laid-open patent publication No. 10-308059 disclose a method of stabilizing surface vibration on an optical disk by rotating a flexible optical disk on a specially provided stabilization plate having a plane surface.
However, with the aforementioned conventional art where the substrate of the optical disk is formed with a rigid body, the optical disk is required to be molded with extreme accuracy for reducing surface vibration and tilt of the high speed rotating disk, and furthermore, the coating of the record film is required to be performed at low temperature for preventing thermal deformation. Such requirements may reduce the yield of products, which may then result to cost rise of optical disks.
Furthermore, with the method of rotating the flexible optical disk on the stabilization plate disclosed in Japanese laid-open patent publication No.10-308059, rotating the optical disk on a simple plane surface causes the optical disk and the stabilization plate to contact and slide against each other. As a result, the optical disk vibrates to thereby cause surface vibration at high frequency. The surface vibration at high frequency often falls in a frequency range for which mechanical focus servo control cannot handle, and therefore, residual servo error cannot be sufficiently eliminated.
Furthermore, surface vibration due to mutual sliding of the optical disk and objective lens causes creation of dirt/dust, and leads to various errors. Particularly, in Japanese laid-open patent publication No. 7-105657 where the record film is positioned on the side facing the stabilization plate, the mutual sliding also causes damage upon the optical disk, and directly results to error in recording/reproducing.
In solving the foregoing problems, Japanese patent application No.2001-228943 discloses an invention where a stabilization guide member is not disposed in a manner facing the entire surface of a recording layer of the optical disk. The stabilization guide member has a cylindrical shape in which a side thereof facing the optical disk is shaped as a circular arc. The stabilization guide member is positioned at a portion where air force serves to stabilize surface vibration, and furthermore, areas that are not affected from the air force (space portions having no stabilizing guide member disposed thereat) are provided on the upstream side and downstream side of said portion stabilizing surface vibration. Accordingly, repulsive force otherwise occurring in the disk can be effectively reduced at said portion since each of said areas positioned before and behind said portion serves as an “escape” which absorbs (or relieves) the repulsive force. Therefore, an increase in the stabilizing effect using air force can be achieved.
With such invention, surface vibration of the flexible disc can be reliably controlled, recording can be performed with high density, and problems such as sliding contact against the objective lens can be prevented. Such invention, however, needs to overcome a critical problem of precisely matching the position of the focal point of the optical pickup with the position for stabilizing surface vibration.
Furthermore, Japanese patent application No.2001-284299 discloses an invention serving to adjust the position for stabilizing surface vibration by controlling the position and the tilt of the stabilization guide, and also serving to match the position of the focal point of the optical pickup with the position for stabilizing surface vibration by measuring the gap between the stabilization guide and the optical disk with use of a gap sensor such as an electrostatic capacity type displacement sensor.
Nevertheless, employing a gap sensor such as the electrostatic capacity type displacement sensor is too costly and is unrealistic to be applied to a typical, widely used optical disk drive.
Meanwhile, Japanese patent application No.2002-12717 discloses an invention serving to control and adjust the positional relation between a stabilizing member and an optical disk according to a result of a detected tracking error signal so that a focus position of an optical pickup and a surface vibration stabilizing position would match with each other.
Although this invention allows the focus position of an optical pickup and the surface vibration stabilizing position to satisfactorily match with each other, the use of this invention is limited to a case where a tracking error signal such as a push-pull signal is detectable in a state where a focus servo is switched off, and is unable to be used when the focus servo is switched on.